With the advent of modern day electronics, the circuits on a PCB (Printed Circuit Board) have become more and more dense. This then requires that the conductor lines be thinner or narrower so that more of them can be placed in a given area. Therefore, the probability of the conductor lines having defects increases and each of the PCBs have to be inspected for faults in the conductor lines. This inspection can be done manually or automatically. Once a fault or defect is found, then it has to be located and repaired. Under presently used methods, the fault or defect is visually located by the operator using a microscope and the conductor is repaired manually.
The testing and repairing of the conductor lines on a Printed Circuit Board or ceramic module are among the most critical steps in the packaging technology. This is because the electronic hardware must be reliable and free from defects, as they are very expensive to manufacture and the field failures cannot be repaired easily. To eliminate these immediate and potential defects, tremendous efforts are being made.
Most defects or faults in a conductor line are due to masking or improper deposition of the conductive material. But they could also be related to other factors, such as impure material or stretching the resolution limits of the lithography process. The most commonly found defects are open and narrow necks.
For an open in the conductor line, the repair method currently being used during a mass production process is to visually locate the open and then braze it manually. One such method is disclosed in U.S. Pat. No. 4,418,264, where a specifically shaped metallic part is placed on the conductor path interruption and by means of micro-resistance welding, the metallic part is welded to the conductor to bridge the interruption. Another method of repairing opens is by decal transfer as disclosed in U.S. Pat. No. 4,704,304.
Laser deposition methods are also being developed for repairing circuit opens. As disclosed in U.S. patent application No. 223,487, filed on July 25, 1988, and presently assigned to IBM, Corp., an open circuit is repaired by laser induced electroplating process based on the thermobattery effect. One tip of the open conductor is heated with a laser beam, and a thermobattery is formed between the hot spot (tip of the conductor) and the cold part (normal section of the conductor). The laser heating of the tip induces the conductive material present in the plating solution to be formed at the hot tip. This process is continued until the growth of the conductive material joins the two open ends of the open, and a continuous electrical path is formed.
However, there are two problems associated with the laser deposition methods. The first is an alignment problem, because the laser beam has to be focused at the site under a microscope, and whether this is done manually or by computer driven processes, both methods are very expensive. The second problem is the laser dosage control. This problem is associated with the fact that the reflectivity of the surface under illumination varies from spot to spot. At some locations it may overheat, and at other locations the heat may not be enough to induce deposition.
For narrow necks and thin portions, more elaborate manipulations are necessary. The repair method currently being used in the industry for the repairs of narrow necks and thin portions is to pass a very strong current pulse (e.g., 50 amperes in 10 micro sec) into each conductor line. The narrow necks or thin portions will burn out by the extreme heat generated there and an open will result. The conductor lines are then visually inspected for circuit opens. The open gaps are then manually cleaned and joined together by methods well known in the art.
Recently non-destructive electrical testing methods (for example, NLC (Nonlinear Conductivity) tester as disclosed in U.S. Pat. No. 4,496,900) have been developed. However, the NLC tester only locates the lines with current constricting defects, such as, cracks, line breaks, intermittent opens, narrow necks, etc., but it does not provide any information as to the exact location of the narrow neck nor does it disclose any method for the repairs of the current constricting defects. Thus, the destructive method as discussed above must still be used. This usage is due to the fact that it is easier to visually locate the burn-outs than untouched defects.
All of the known processes are also very time-and-money consuming. This is because the defective spot has to be visually located and only then can it be repaired. However, the method of this invention is much more efficient. The defective site in the conductor line having a thin portion or a narrow neck does not have to be physically located to initiate the repairs, and the repairs that are made are similar or better than the ones obtained in the prior art. The process of this invention is self-induced, i.e., the passage of the high current creates a hot spot at the defective site and conductive material is induced to be deposited at the defective site. The process of this invention is also self-limiting, i.e., when the defect has been repaired, the process will slow down and stop by itself. The method of this invention also reduces production cost considerably.